Most of the conventional semi-submersible structures comprise a hull that has sufficient buoyancy to support a deck box or platform above the water surface. The hull typically comprises two substantially parallel pontoons and a plurality of vertically upstanding columns that extend from the pontoons to support the deck box above the water surface. The pontoons and portions of columns are submerged below the operational water line during norming operation.
FIG. 1 shows one of such conventional semi-submersible structures. The semi-submersible structure shown in FIG. 1 includes a plurality of main columns (1) and a plurality pencil columns (2) that support a deck box (3) above the water surface. The pencil columns (2) are provided to provide the semi-submersible with better motion control and as a result, higher uptime. Each of the main columns (1) consists of a fork shaped wave diverter (4) provided at one surface of the main columns for reducing the wave drift force on the semi-submersible. Although conventional semi-submersibles of such configuration may have acceptable motion responses in normal weather condition, their motion responses during severe weather conditions are typically excessive and unacceptable for some applications, that is, they still face a relatively large heave, pitch and wave motions when operating under severe weather conditions.
The conventional semi-submersible structure shown in FIG. 1 also includes a plurality of bracings (5). The bracings are to provide adequate structural integrity to the structure. However, such bracings often create undesirable characteristics, for example, hydrodynamic drag and create problems related to underslung loads.
Other challenges face by conventional semi-submersibles when operating under harsh offshore environmental conditions include unsymmetrical load distribution of the semi-submersible which makes it not ideal for mooring design. In particular, the total wind and current load in quartering sea could be about 40% more than at the head sea. The total wind and current load in the beam sea could be about 20% more than the loads at the head sea. As a result, the mooring design of a semi-submersible is greatly influenced by forces from the quartering sea. A conventional semi-submersible may utilize a mooring system that consists of a 12 point chain and 4 thrusters. This results in unsymmetrical load distribution on the semi-submersible and this in turns, makes it not ideal for mooring design.
Another conventional semi-submersible structure known in the art is one that comprises a ring pontoon, a plurality of vertically upstanding columns that extend from the pontoons to support a rectangular deck box above the water surface. The semi-submersible of this configuration has several drawbacks. One of which is that it has high mass due to large displacement of the semi-submersible and high added mass due to the semi-submersible's large skirts. The semi-submersible has a relatively high natural period and this shifts the Response Amplitude Operator (RAO) curve to the right. However, the mono-hull of the semi-submersible of this configuration does not have any heave second hump.
It is therefore desirable to provide a semi-submersible structure that seeks to address at least some of the problems encountered in conventional semi-submersibles, or at least to provide an alternative.